Why can't we call it a mental problem?

We can, because it is.

I wanna know how someone concludes they are the wrong gender. If you like things that guys like then you’re a guy, regardless of biology? Or if you “feel” like a guy. How would a woman know what being a guy is supposed to feel like? They’d have no way of knowing.

It's in their mind, so why is it mean to acknowledge that?

Birth defects and mental illness are two very different things.

There is a book called ‘as nature made him’. It is the true story of a guy named David, who, in the 1960’s had his penis essentially burned off during a botched foreskin removal. The doctor convinced his parents to give him sex reassignment surgery as an infant and raise him as a girl to remedy the problem.

But he grew up his entire life knowing that he was not a girl and when he found out the truth, it ultimately drove him to suicide.

So, no matter what body you have or how people raise you, you know who you are inside.
If he knew in his mind he was a boy, even though he now physically had girl parts and was being given girl hormones, it makes sense that others would know too.

It's not a mental problem. It's due to some hormonal imbalance during pregnancy, that feminized the brain in males or masculinized it in females. Once the brain is masculinized or feminized during pregnancy, that's set in stone and won't change until you die.

Gender Dysphoria is what happens when the body is male but the brain is female (or the opposite). It's not an indicator of being mentally ill: it's something that happens to everybody when body and brain gender don't match. During the 70s, some kids were operated at birth because of birth physical problems that screwed the male genitalia. They were operated to have female genitalia and given hormones. They were raised as females. Most of them developed Gender Dysphoria, which is the reason they stopped doing it.

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If you want to be butch, be butch.
If you want to be fem, be fem.
That's YOU acting out how you "feel."
But, don't tell me you ARE the opposite sex.
It's too difficult to understand. And, you'll never be the opposite sex.

We should just split the difference and allow the gender-confused be honorary members of the other sex if they so choose. It'd be like how honorary military ranks are sometimes given to people who've never served, or honorary college degrees to celebrities who never attended the university. But allowing honorary transgenders to compete in sports against their opposite born sex, forget about it!